[time-nuts] Low noise quartz crystal oscillator by Bruce Griffiths

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Oct 28 01:11:46 EDT 2015


Surely you meant to write

PN(SSB) = -177 -Pout + NF

If we attempt to apply this equation to the 10811A for which you measured a PN floor of -174dBc/Hz
this implies that 

NF - Pout = 3dB

Best case (NF = 0dB - unlikely! Pout would need to be much higher for nonn zero NF)

Pout =-3dBm or 500uW.

The question is identifying this power.
The crystal dissipation is 50uW (HP Journal March 1981 p24)
The signal power dissipated in the CB stage input R is around 10% of this or about 5uW.

The answer to this conundrum is surely that the equation for PN doesn't apply directly in this case 
for offset frequencies outside the crystal bandwidth.
The Crystal actually bandpass filters the signal and PN noise generated by oscillator.
For offset frequencies outside the crystal bandwidth the oscillator generated PN is greatly attenuated 
so that the noise of the buffer amplifier chain (CB stage plus output amplifiers) dominates.
In calculating the noise floor of the buffer amplifier chain the fact that the crystal has 
a high impedance at these frequencies should be taken into account.     


     On Wednesday, 28 October 2015 8:34 AM, "KA2WEU at aol.com" <KA2WEU at aol.com> wrote:

 I have bought and measured the hp10811 at about -174dBc/Hz. The interesting  thing is  the feedback capacitor from collector to base which changes Rin=1/gm. Unless the circuit has a hidden Q mulitplier the PN (SSB) can never be better then 177 (kT) in dBm  + Pout  in dBm - NF of the oscillator transistor. Many of the GB stages are potentially unstable , so the "hopeful' best PN (SSB) is 177dbm + Pout ! AT 100 Mhz the leaing values are -146/100Hz offset  and - 183 far out and high crystal dissipation, 2mW or so  Ulrich  In a message dated 10/27/2015 4:17:16 P.M. W. Europe Standard Time, bruce.griffiths at xtra.co.nz writes:
As Rick has pointed out numerous times when the output signal is extracted via the crystal by a CB stage (or cascade thereof) the PN floor is determined by the ratio of the amplifier equivalent input noise current to the crystal current. That is the amplifier equivalent input noise current at frequencies for which the crystal impedance is high. If one neglects this crucial point one comes to the conclusion (e.g. see Eq 4.-1 page 274 of Ulrich Rohde's: Microwave and Wireless Synthesisers Theory and Design.) that with a crystal current of 1.4mA rms and a crystal esr of 50 ohms that the XO PN floor cannot be lower than -154dBc/Hz.  Even the XO circuit in the ARRL handbook (attributed to Ulrich) using this method of signal extraction has a measured PN floor of -168dBc/Hz.  Many other XO's (including the 10811A which uses a crystal current of 1mA ) have an actual PN significantly lower than this.  One would have thought that this glaring discrepancy between "theory" and practice would have been noticed and corrected by now.

     On Tuesday, 27 October 2015 6:01 PM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:

The oscillator transistor and buffer amplifier are basically
the same as the HP 10811, except for the absence of mode
suppressors.  The difference here is that the oscillator
self limits in the oscillator transistor, whereas the 10811
has ALC.  The discontinuous operation of the transistor,
as explained by Driscoll some 45 years ago, is undesirable
because it increases the load resistance the crystal sees.
The 2 transistor "Driscoll oscillator" fixes this problem
by using an additional stage that limits instead of the
oscillator transistor.  This has been widely used for
decades.  It is interesting to note that the 10811 ALC
works by varying the DC bias current in the oscillator
transistor.  This is in contrast to the elaborate DC
bias current stabilization here.

I have demonstrated that the close in phase noise in
the 10811 is entirely due to the flicker noise of the
crystal.  The only place where the 10811 circuit comes
into play is beyond 1 kHz from the carrier, where the
Burgoon patent circuit (which apparently has prior art
from Ulrich Rhode) reduces the phase noise floor.  I
have built two different oscillator circuits for 10811
crystals and have measured the flicker noise as being
the same as the intrinsic noise of the crystal.

Thus, obsessing over noise in oscillators circuits may
be overkill, unless you are planning to use a much
better crystal (BVA, etc).  OTOH, it might be advantageous
to improve the reverse isolation by adding additional
grounded base buffer stages.  There are various NBS/NIST
papers where several grounded base stages are cascaded.
I did this in the HP 10816 rubidium standard.

It is good to see time-nuts learning about oscillator
circuit by building them.

Rick Karlquist N6RK
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